The present invention relates to heliostat assemblies, and more particularly to such assemblies used in electro-optical systems that are preferably adapted to be mounted in pods mounted from an air frame. The present invention is more particularly directed to the half angle drive system associated with a gimbaled mirror utilized in an electro-optical system for line-of-sight direction control. The function of an airborne heliostat electrooptical system is to provide a space stabilized line-of-sight direction capability relative to the movement of the aircraft.
A prior art stabilization system for stabilizing and maintaining a fixed line-of-sight of an optical instrument on an aircraft is described in U.S. Pat. No. 3,518,016. Such stabilization system utilizes a gimbal mounted mirror with a direct drive DC torque motor, and a pulley driven half angle drive mechanism which is gyro controlled. In other prior art, a stabilization sighting is set forth in U.S. Pat. 3,446,980, and deviations from the line-of-sight of an electro-optical system are sensed relative to a gyro spin axis, and correction signals are generated to drive the mirror and maintain the line-of-sight without the use of mechanical transfer mechanisms. Such system utilizes an autocollimating mirror to sense the deviations from the gyro spin axis, and includes correction signal means which are applied by a closed loop servo means to cause the line-of-sight to correspond to the gyro spin axis. Such electric mirror drive also utilizes half angle mechanisms for driving the mirror, but has the disadvantage of requiring a larger volume to accommodate the autocollimation function and required alignment difficulty. Such autocollimated systems have volume and configuration disadvantages because of the need for a retroreflector on the stabilized body, and the multiple optical paths required to generate the correction signals. As a result, the ratio of gimbal size to useful aperture is poor, and is generally a more complex system requiring processing of optical information to determine the error signal required to null the servo.
A heliostat which utilizes a two axis gimbal set with an elevational gimbal structure which is annular, and wherein an azimuth gimbal is also an annular structure, is set forth in copending application Ser. No. 268,546, filed May 29, 1981 and entitled "Improved Inertialy Dampened Heliostat Assembly", which application is owned by the assignee of the present invention. In such copending application, a belt driven half angle drive system is utilized which includes an inertial dampener fastened to the mechanical pulley drive mechanism which rotates the mirror. A further showing of typical half angle mechanical drive systems utilized in heliostats is seen in U.S. Pat. No. 3,951,510, also owned by the assignee of the present invention.
The aforementioned U.S. Pat. No. 3,951,510 and copending application both describe a mirror gimbaled in two axis for line-of-sight direction and stabilization in a pod mounted electro-optical system utilizing a mechanical half angle drive system for the mirror. A continuing source of angular error in such systems is due to the vibrational peaking of the mechanical drive system which is in part due to vibrational modes of the gimbal coupled to the mechanical half angle mirror drive. Error inducing vibration can result from air frame and pod vibration modes which are at the lower end of the frequency spectrum and result in base rotation of the gimbal set. It is this base rotation which must be exactly halved by the half angle mirror drive to maintain a fixed line-of-sight. Gimbal vibration modes cause mirror rotation without base rotation and thus serve to jitter the line-of-sight. An ideal mirror drive will respond to the base rotation with a conventional half angle correction, but reject motion caused by high frequency gimbal vibration modes. It is therefore desired to decouple the mirror shaft from the gimbal while continuing accurate response to base rotation.